[Merged by Bors] - atx: cache poet proofs with lru

[dshulyak]

followup for https://github.com/spacemeshos/go-spacemesh/pull/6326

in current code poet proofs are fetched for every atx submitted to the node, they are relatively large (140KB) and account for sizeable chunk of all reads executed on the atx handler codepath (25%).

they are also a perfect case for lru caching, they are mostly reused and replaced from epoch to epoch.

basic stats in recent epochs.

select round_id, count(*), max(length(poet)) from poets group by round_id;
25|42|146200
26|45|145936
27|45|145738
28|45|145903

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.50000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 81.7%. Comparing base (87893ee) to head (d71bc8d). Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
activation/poetdb.go	66.6%	10 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #6336 +/- ## ========================================= - Coverage 81.8% 81.7% -0.1% ========================================= Files 312 312 Lines 34606 34637 +31 ========================================= + Hits 28314 28328 +14 - Misses 4461 4472 +11 - Partials 1831 1837 +6 ```

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[dshulyak]

@poszu updated with singleflight just in case, could you please check again?

[dshulyak]

bors try

[spacemesh-bors[bot]]

try

Build failed:

• systest-status

[dshulyak]

bors merge

[spacemesh-bors[bot]]

Pull request successfully merged into develop.

Build succeeded:

• ci-status
• systest-status